Smith U.S. Pat. No. 3,356,542 granted Dec. 5, 1967 (the "Smith patent") is directed to cobalt-nickel base alloys containing chromium and molybdenum. These alloys are said to be corrosion resistant and capable of being work-strengthened under certain temperature conditions to have very high ultimate tensile and yield strengths. The patented alloys can exist in one of two crystalline phases, depending on temperature. They are also characterized by a composition-dependent transition zone of temperatures in which transformations between phases occur. At temperatures above the upper temperature limit of the transformation zone, the alloys are stable in the face-centered cubic ("fcc") structure. At temperatures below the lower temperature of the transformation zone, the alloys are stable in hexagonal close-packed ("hcp") form.
By cold working metastable face-centered cubic material at a temperature below the lower limit of the transformation zone, some of it is transformed into the hexagonal close-packed phase which is dispersed as platelets throughout a matrix of the face-centered cubic material. It is this cold working and phase-transformation which is indicated to be responsible for the ultimate tensile and yield strengths of the patented alloys.
It is characteristic of the Smith patent alloys that they are relatively expensive because of their high content of components such as nickel, molybdenum, and cobalt, and relatively low content of alloy components of lesser cost, such as iron. Iron may be present in the Smith patent alloys in amounts only up to 6% by weight for example.
In response to the demand for alloys less expensive than those of the Smith patent, the alloys disclosed in Slaney U.S. Pat. No. 3,767,385 granted Oct. 23, 1973 (the "Slaney patent") were developed. The alloys disclosed include elements, such as iron, in amounts which were formerly thought to result in the formation of disadvantageous topologically close-packed phases such as the sigma, mu or chi phases (depending on composition), and thus thought to severely embrittle the alloys. But, this disadvantageous result is said to be avoided with the invention of the Slaney patent. For example, the alloys of the Slaney patent are reported to contain iron in amounts from 6% to 25% while being substantially free of embrittling phases.
According to the Slaney patent it is not enough to constitute the patented alloys within the ranges of cobalt, nickel, iron, molybdenum, chromium, titanium, aluminum, columbium, carbon and boron specified. Rather, the alloys must further have an electron vacancy number, (N.sub.v), which does not exceed certain fixed values in order to avoid the formation of embrittling phases.
By using such alloys, the Slaney patent states, cobalt-based alloys which are highly corrosion resistant and have excellent ultimate tensile and yield strengths can be obtained. These properties are disclosed to be imparted by formation of a platelet hcp phase in a matrix fcc phase. This is accomplished by working the alloys at a temperature below the lower temperature of a transition zone of temperatures in which transformation between the hcp phase and the fcc phase occurs.
Another alternative is the alloy described in Slaney U.S. Pat. application Ser. No. 893,634, filed Aug. 6, 1986 (the "Slaney application"), which is a continuation of U.S. Pat. application Ser. No. 638,985 filed Aug. 8, 1984 (now abandoned). The alloys disclosed in the Slaney application are said to retain satisfactory tensile and ductility levels and stress rupture properties at temperatures of about 1300.degree. F. (700.degree. C.). The alloys contain substantial amounts of cobalt, chromium and nickel, a maximum of 1 percent by weight iron, and optionally small amounts of titanium and columbium as well. In order to avoid formation of embrittling phases, such as the sigma phase, it is also disclosed that the electron vacancy number for the alloys disclosed in the Slaney application be no greater than 2.8. Again, the alloys are disclosed as being strengthened by working at a temperature which is below that the lower temperature of a transition zone of temperatures in which transformation between the hcp phase and the fcc phase occurs.
It is believed clear that strengthening of the alloys of the foregoing patents and application is attributed to cold working causing formation of hcp platelets in the fcc matrix, and optionally a subsequent heat-aging at a somewhat elevated temperature--for instance cold working, to obtain an approximately 5 to 70% reduction in thickness, and subsequent aging in the temperature range of 426.degree.-732.degree. C. for about 4 hours. There is no mention in any of the Smith and Slaney patents and Slaney application that strengthening should be achieved by formation of gamma prime phase in the alloys. However, as will be seen, the present invention is premised upon the recognition that advantageous mechanical properties (such as high strength), and high hardness levels, can be attained in certain alloy materials having high resistance to corrosion through formation of a gamma prime phase in those materials and the retention of a substantial gamma prime phase after the materials have been worked to cause formation of an hcp platelet phase in an fcc matrix.